newbie question

Dear users..
I have fastly created a table in a postgresql database..
some columns where edited by hand (columns A, B, C), and some others
(columns D, E, F) have been calculated as a result of mathematical
equation (where the factors are the A, B, C columns)....
now I simply need to change some values in the A, B, C columns and I
would like to obtain the correct values in the D, E, F column...
I know that this is a tipical problem of a spreadsheet but how can I
solve it with a DBMS??
there a way to impose some constrain, also after the table is already
filled with values?? 
I need only some suggestions to start.. then I can go on by myself!!
thank you very much
Ivan



-- 
Ivan Marchesini
Department of Civil and Environmental Engineering
University of Perugia
Via G. Duranti 93/a 
06125
Perugia (Italy)
e-mail: marchesini@unipg.it       ivan.marchesini@gmail.com
tel: +39(0)755853760
fax: +39(0)755853756